Extreme rainfall is often endangers public health, safety and causes significant economic losses. When occurring in metropolitan areas, such events especially cause more serious damages due to the high concentration of density. The historical rainfall amount recorded near Busan metropolitan area on 7 July 2009. An extreme rainfall-producing quasi-stationary mesoscale convective system (MCS) developed along the Changma front, producing 362 mm of rain for 12 h over the Busan metropolitan area. This event caused significant damages to the Busan metropolitan area, as well as flooding and landslides in the southern Korean Peninsula.
With moist conditionally unstable in its environment, the continuous upstream initiation of deep convection (back-building process) occurs at the coastal line while older convective cells move downstream parallel to the convective line (train effect), resulting in large local rainfall amount. The Cloud-Resolving Storm Simulator (CReSS) model of Nagoya University (Tsuboki and Sakakibara 2002) was performed to examine the reason of upstream initiation of deep convection. The model could reproduce quasi-stationary MCS and simulated new convective cells repeatedly formed on the southwest end of convective line. The occurrence of convection is initiated due to colliding warm southwesterly and cold northeasterly flows. Strong warm and moist air was transported by southwesterly monsoonal flow and the downdraft-driven cold air depicted northeasterly flow within outflow boundary. The sensitivity experiment shows the effects of hydrometeor evaporation, the cooler northeasterly flow is to be enhanced by evaporative cooling. It plays an essential role in maintenance of quasi-stationary MCS over Busan metropolitan area.